An OLED is a thin-film LED including a light emission layer formed of an organic compound, and uses the phenomenon of electroluminescence in which a light emission layer emits light when a current flows through the fluorescent organic compound. Such an OLED may be driven at a low voltage and manufactured as a thin film type, and has a wide viewing angle and a rapid response speed. Therefore, even when the OLED is seen from a side, the quality of the OLED is not changed, and an afterimage does not remain on a screen, different from a liquid crystal display (LCD). Furthermore, full coloration may be achieved. Accordingly, the OLED has a great potential to become a leader among the next-generation flat panel display devices.
In general, an OLED includes an anode (ITO layer), a hole injection layer, a hole transport layer, a light emission layer, an electron transport layer, an electron injection layer, and a cathode, which are sequentially stacked on a transparent substrate. When power is supplied thereto, electrons are transferred into the light emission layer by the assistance of the electron transport layer at the cathode, and holes from which a electron is escaped are transferred into the light emission layer by the assistance of the hole transport layer at the anode. The electrons and the holes which join in the light emission layer formed of an organic material generate excitons having high energy. The excitons generate light while the energy of the excitons is lowered.
Meanwhile, such an OLED is known to have low light extraction efficiency. The main reason that the light extraction efficiency is low is the occurrence of total internal reflection in which light is totally reflected into the OLED at the interface between the transparent substrate and the anode (transparent electrode) having a large refractive index or between the transparent substrate and the air. Therefore, in order to substantially prevent such total internal reflection and increase the optical extraction efficiency of the OLED, various methods have been proposed, including forming irregularities on the transparent substrate of the OLED or attaching a cone-shaped light extraction sheet to the transparent substrate. When such methods are used, the light extraction efficiency thereof is somewhat improved. However, the improvement effect is too slight to achieve a sufficient effect.